3 Answers
3

If the equipment is grounded, this could be a ground loop. (If you're using AC power and three-prong plugs, the third prong is the ground.) Try plugging the amp into a different outlet, one that's on a different circuit. They make ground loop isolators, but I don't know if they work or not; it might be worth investigating.

(I've used three-to-two-prong adapters to get rid of ground loop problems in a pinch, but I don't recommend it as more than a temporary solution. As per Owen's comments, it's quite dangerous to do this.)

A word of caution, as noted above, removing the earth on any device with an earth pin is inviting danger. If it was a guitar amp for example, as the person holding the guitar, and thus touching the strings you become the earth.
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Owen KellyMay 17 '11 at 14:45

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@Owen - I agree, not something to be done with regularity or with delicate equipment. Could this cause a hazard if guitars aren't involved?
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neilfeinMay 17 '11 at 15:34

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It would depend on the nature of the device. By removing the ground, any charge that would normally pass through the ground (such as an electrical fault), will now need to find a new exit. If you are touching any part of the device which could be connected to the ground (such as the chassis, or metal enclosure), then you will become the ground. Touching the strings on an electrical guitar (for an unearthed amp), or holding a microphone (for an unearthed mixer) could result in some fatal earthing. Try a Ground Loop Isolater instead.
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Owen KellyMay 18 '11 at 8:56

From my experience, sometimes it does. I believe it should be inline on the signal line, though I have only ever tried this on a dc player, turntable. Not on an instrument. The theory is similar I think, but I'm not 100%.
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Owen KellyMay 20 '11 at 11:26

I was told one of the major reasons for balanced cables (note most guitar cables are unbalanced), is to prevent interference. Most notable is that of radio transmissions being picked up. The length of the cable relates to the frequency it will pickup, though your amp, or pickups could also pick up the signal.

Proper shielding, and shorter cables should solve the issue.

If your keyboard and amp can take a balanced cable investigate how to achieve this, as it should solve your issues.